fstests: get section config after RUN_SECTION checks

fstests: get section config after RUN_SECTION checks

While trying to do

./check -s <some section>

I was failing because I had a section defined higher than <some section>
that had TEST_DEV=/some/nonexistent/device, since I was using the other
section to test an experimental drive.  This appears to be because we
run through all of the sections, and when getting the section config we
check to see if it's valid, and in this case the section wasn't valid.

The section I was actually trying to use was valid however.  Fix check
to see if the section we're trying to run is in our list of sections to
run first, and then if it is get the config at that point.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>